The invention relates to polyolefin electrical insulation having an additive for retarding water trees which is useful for cables and wires carrying medium and high voltage such as about 10 kV or higher.
In electrically stressed polyolefin insulation, processes can occur which are called "electrochemical treeing" (ECT) or "water treeing". These processes, which are of importance especially in view of the operating safety of plastic-insulated medium and high-voltage cables, lead to the development of branching or tree-like structures, the so-called ECT structures.
The appearance of ECT structures which are visible with particular contrast and detail after suitable staining, varies greatly. In principle, two forms are distinguished:
(a) "vented trees" which start from the surface of the insulation and extend into the insulation, and
(b) "bow-tie trees" which are generated in the interior of the insulation.
The mechanism of ECT development has not yet been clarified. However, it is generally assumed that an electric field and the presence of a polar liquid, especially water, are required for the formation of ECT structures; the ECT structures are therefore also called water trees. The points of initiation of the water trees always seem to be faults in the insulation such as impurities, aggregated admixtures, voids, gaps, cracks or boundary surfaces. These faults however, only partly lead to the formation of water trees. From the faults, which in mass-produced insulation cannot be avoided completely, the tree-like structures extend in the direction of the electric field.
Since ECT structures represent local changes of the insulating material, they can cause damage to the insulation, especially with respect to its dielectric strength. Numerous attempts have therefore already been undertaken to prevent or at least retard the growth of water trees.
One of the measures to prevent the formation of water trees is to provide the insulating layer with a metallic envelope or jacket, for example, of lead or aluminum. Cables with such a water shielding layer, however, are not only more expensive but are also heavier and therefore more difficult to handle than cables without metal jackets.
For this reason it has been attempted to prevent the formation of ECT structures by additives to the insulating layer (or adjacent layers). From the numerous compounds used for this purpose, the following are mentioned as examples: lead stearate (DE-OS No. 24 25 760 or British Pat. No. 1,473,867), sodium chloride and sulfate or other strong electrolytes (DE-AS No. 25 37 283 or U.S. Pat. No. 4,042,776), salts forming stable hydrates such as calcium and magnesium chloride, and basic anhydrides (DE-OS No. 28 17 804 or British Pat. No. 1,584,501), silica gel and phosphous pentoxide (DE-OS No. 27 54 336: page 6, last paragraph), organosilanes (U.S. Pat. Nos. 4,144,202; 4,212,756 and 4,263,158 as well as DE-OS No. 28 05 875: page 3, paragraphs 2 and 3), lead oxides and basic lead compounds (DE-OS No. 25 23 844: claim 1, and DE-OS No. 28 06 752: claims 1 and 3), organic isocyanates (U.S. Pat. No. 4,282,333), silane compounds grafted on to the polymer material (DE-OS No. 29 35 224; claim 1) and metal complexes of diketones, salicylic acid or Schiff bases (European Pat. No. A1-27 300).
On the other hand and contrary to the proposal to add salt-like compounds or electrolytes, it has been proposed to keep the content of water-soluble and/or hygroscopic salts finely distributed in the insulation below a value of 10.sup.-1 ppm and preferably under 10.sup.-4 ppm (DE-OS No. 29 11 756: page 3, paragraphs 3 and 4).
However, all these measures, which are in part contradictory, have not yet led to the desired success. It is recommended in a report on CIGRE, the International High-Voltage Conference of 1980, (see: "Kunststoffe" 71, 1981, pages 448 sff, particularly on page 450 to page 451), that a water-tight metal shield should be applied to (high-voltage) cables in order to prevent moisture from penetrating.
It is therefore, an object of the invention to suppress effectively and permanently in electrical insulation composed of cross-linked and/or linear polyolefins, the growth of water trees or to prevent their substantial formation through the use of additives so that expensive measures such as metal jackets need not be used.